For the past 40 years a gung-ho group of astronomers has been sweeping the skies with radio telescopes in the hope of stumbling across a message from an alien civilisation. Known by its acronym of SETI - Search for Extraterrestrial Intelligence - this enterprise gained popular attention with the Hollywood movie Contact, starring Jodie Foster. It makes good science fiction, but is it good science?

The eminent biologist George Simpson once described SETI as "a gamble of the most adverse odds in scientific history". Even optimists concede it is a needle-in-a-haystack quest. So far, there isn't a shred of convincing evidence for any life beyond Earth, let alone intelligent life. With a hundred billion stars in our galaxy alone, and a billion possible radio channels to choose from, pointing the radio telescope in the right direction and tuning into ET's chosen station might take a very long time - even supposing there is anybody out there transmitting.

So why do it? Why waste time and resources on such a speculative venture? I agree that SETI is almost certainly a hopeless enterprise, but it is also a glorious one that is definitely worth doing. This seems like a contradiction, but it isn't.

For a start, it costs the taxpayer nothing. The main projects are managed by California's SETI Institute and the grassroots international SETI League, both of which are entirely privately funded. Second, it has a strong educational appeal. Aliens are a favourite topic with schoolchildren: witness the runaway success of seti@home - a way to use your screensaver to scan telescope data for alien signals.

More significantly, SETI forces us to confront some of the deepest scientific and philosophical questions of our time. Frank Drake, the American astronomer who started the project in 1960, is fond of saying that the search for life beyond Earth is really a search for ourselves, who we are and where we fit into the great cosmic scheme of things.

Speculations about whether or not we are alone in the universe go back at least to ancient Greece. Today, most people - including many scientists - instinctively believe there must be life out there somewhere. Examining the assumptions that underpin this belief is very revealing. For example, the origin of life on Earth remains mysterious. It could well have been the result of a stupendous chemical fluke. Computing the raw odds quickly shows that even the simplest known cell is so unlikely to form by accident it wouldn't happen twice in the entire observable universe. Or in a trillion similar universes. One near-miracle is possible, or we wouldn't be here to comment on it. But two near-miracles?

Perhaps life's origin wasn't a freak event after all, but the automatic outcome of inherently bio-friendly laws of nature. However, the laws we know certainly don't have "life" written into them. In fact, the very notion of a law of nature is that it applies across the board to everything. Life is a weird and exceedingly special state of matter, and it's hard to see how a basic law of nature could know anything about it in advance. Yet that hasn't stopped the distinguished physicist Freeman Dyson from claiming that somehow the universe "knew we were coming".

Dyson echoes a widespread sentiment. Belief that there is an inherent cosmic drive from matter to life permeates much scientific thinking. But it is rarely articulated explicitly; after all, if life pops up wherever there are earthlike conditions, then there seems to be something deeply contrived in the way the universe is put together. SETI obliges us to unpack that extraordinary claim and face the fact that if there is a law that steers matter to life then we haven't found it yet, and it will be a law like no other we have discovered in nature so far.

Similar issues swirl around the question of intelligence. A popular conception of evolution is that, over time, life progresses from simple to complex, marching inexorably onwards and upwards, continually striving for advancement. Biologists flatly deny this. The essence of Darwinism is that nature is blind and evolution is directionless. There is no known principle that compels life to evolve toward intelligence once it gets started. But belief in alien civilisations tacitly assumes a thrust towards intelligence, a hidden directionality in evolution, which is sharply at odds with the whole spirit of Darwinism.

If SETI draws a blank in, say, a hundred years, the effort will not have been wasted. Although one can't prove a negative, decades of unsuccessful searching would lead many people to conclude that we are, after all, probably alone in the vastness of the cosmos. That conclusion would give added urgency to our responsible stewardship of planet Earth. If humans are the only organisms in the universe capable of reflecting on the significance of their own existence, then our unique planet would be seen as a truly cosmic resource.

Paul Davies is professor of natural philosophy at the Australian Centre for Astrobiology at Macquarie University in Sydney. This article is based on his Michael Faraday Prize lecture The Origin of Life, given at The Royal Society on January 27, 2003.

Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.